Start cleaning up and fixing smime_keys.pl (closes #3324) (see #2456)

Start cleaning up and fixing smime_keys.pl (closes #3324) (see #2456)

* Convert to using File::Temp (#3324).  This was also suggested at
  https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=775199

* Use File::Temp for add_p12 temp file. (#2456)

* Make the query_label() method a bit more robust with empty strings,
  ctrl-d, and leading spaces.

* Clean up openssl_do_verify() logic.  Mark cert as invalid
  rather that die'ing if an openssl verify command fails.

* General cleanup:
  - Clearly separate op handler, certificate management, and helper
    functions by section and using prefixes.
  - Create openssl helper functions to reduce copy/paste invocations
    and make the code clearer.
  - Make indentation consistent at 2 spaces.
  - Change handle_add_pem() to re-use handle_add_chain() once the
    correct files are identified.
  - Change openssl_parse_pem() to return a single array of data
    structures representing the parsed certs/keys.